Semiconductor devices packaged in plastic or resin encapsulants typically include portions of a lead frame. More specifically, the frames include a plurality of leads electrically coupled to a semiconductor die and a support member, referred to as a die pad, on which the die is mounted. In many instances, the die pad is configured as a solid plate that is slightly larger in area than the semiconductor die and made of the same material as the other portions of the lead frame, for example, copper, a copper alloy, or a plated material. In other instances, the die pad is configured as a circular or square-shaped plate of substantially smaller area than that of the semiconductor die. Each of the conventional types of die pad configurations has drawbacks and disadvantages, as will be described infra.
An example of a conventional die pad configuration is shown in FIG. 1 in simplified form for illustrative convenience, and comprises lead frame 10 having top and bottom spaced apart parallel strip-shaped rails 11 and 12, with tie bar 13 extending substantially perpendicularly therebetween and including a paddle-shaped die pad 14 in a central region thereof. Die pad 14 can be integrally formed with the tie bar 13 or can be separately formed and affixed to the tie bar by any conventional method. In addition, lead frame 10 includes a plurality of inner leads 15 and outer leads 16 integrally formed with each other and connected by dambars 17. The arrangement of inner leads 15 defines an area 18 for receiving a semiconductor die 19 supported on die pad 14. As illustrated, semiconductor die 19 is slightly larger in area than the underlying die pad 14.
In order to assemble a semiconductor package, a bonding material, such as an epoxy-based adhesive, is conventionally applied to the exposed upper surface of the die pad or to the lower surface of the semiconductor die and subjected to a curing process utilizing a clamping device for applying heat and/or pressure to the die pad/adhesive/semiconductor die composite to effect bonding between the die and the die pad. Thereafter, a wire bonding step is performed to electrically connect the inner leads 15 of the lead frame 10 to bond pads on the upper surface of the semiconductor die by wires 20, typically comprising gold, aluminum or alloys thereof.
Subsequent to lead bonding, the assembly is subjected to a plastic encapsulation process in which the lead frame/bonded die assembly is held within a mold into which a molten mold compound (polymeric resin) is injected and subjected to a curing process, such that the outer lead portions extend outside of the molded body. The extending portions of the outer leads are then bent into a desired configuration for use.
In addition to the obvious drawbacks of the above die pad configuration, such as the large amount of metallic material required for the paddle-shaped die pad, along with a correspondingly large adhesive requirement, several additional drawbacks are incurred with the FIG. 1 configuration. For example, since the edge of the die pad is close to the inner leads, adhesive material which is squeezed out around the edges of the pad as a result of compression and increased fluidity during the bonding/curing process, contacts and contaminates the inner leads or flows upwardly toward the bond pads on the upper surface of the semiconductor die, thereby deleteriously affecting device performance, especially when the adhesive is not sufficiently insulative.
Another, less apparent drawback attendant upon the use of large area die pads is package cracking of plastic encapsulated semiconductor devices, which arises from a combination of factors. One factor is internal delamination between the plastic encapsulant material and the die pad of the lead frame. The lead frame and associated die pad typically comprise copper or a copper alloy and, therefore have a coefficient of thermal expansion (CTE) which in most instances is different than that of the surrounding encapsulant material. As a result of this CTE mismatch, stress is created at the encapsulant/die pad interface as the semiconductor device experiences temperature changes. Upon reaching a maximum threshold, the stress is relieved through delamination of the encapsulant/die pad interface.
Another factor associated with delamination in the paddle-type die pad configuration is poor adhesion between the die pad and conventional adhesive epoxies used to attach the semiconductor die to the die pad. Typically, an epoxy adhesive is applied to the die pad of the lead frame. Upon bonding the semiconductor die to the die pad, the epoxy is dispersed, such that it forms a thin, continuous layer beneath the entire die. While conventionally employed epoxy adhesives bond well to the surface of the semiconductor die, adhesion between the epoxy and the die pad is not as strong. Therefore, under certain stress conditions, delamination between the die and the die pad occurs.
Several prior approaches to reduce internal delamination involve increasing adhesion between the die pad and the plastic encapsulant. In one such approach, the surface of the die pad is roughened to increase the surface area available for bonding. In another such approach, the die pad area is reduced relative to the area of the semiconductor die to provide a correspondingly greater area of plastic encapsulant/semiconductor die interface. However, the latter approach, whether employing, for example, a small square-shaped die pad 14 supported by a single, very narrow tie bar 13, such as shown in FIG. 2, or a small circular-shaped die pad 14 supported by at least one tie bar 13, as shown in FIG. 3, disadvantageously lacks sufficient robustness. For example, as shown in FIG. 4, lifting of die pad 14, separation (delamination) of die 19 from bonding adhesive 21, and/or upward bowing of tic bar 13, as shown in FIG. 4, occurs during wire bonding or other processing prior to encapsulation. Such lack of robustness and propensity for pad lifting and tie bar bowing are attributed to the very small area of die pad attachment to the semiconductor die and insufficient tie bar stiffness.
Yet another drawback associated with the use of die pads supported on tic bars of insufficient mechanical robustness, whether the die pad is large or small, is illustrated in FIGS. 5 and 6. Referring to FIG. 5, a lead frame comprises top and bottom rail portions 11 and 12, respectively, which support die pad 14 via tie bars 13. A semiconductor die 19 is bonded to die pad 14 and the inner leads (not shown for illustrative simplicity) are connected in a conventional manner. Thereafter, the assembly of the die 19, die pad 14, inner leads, and inner portions of the tie bars 13 are molded together using a plastic molding compound or resin 21. Molding is conducted by holding the semiconductor die 19, die pad 14, and tie bars 13 in position using a mold having a molding material injection port 22 and a cavity 23 communicating with injection port 22. Molding material 21, in a molten state, is injected within the cavity (indicated by arrows) to fill the entire cavity 23..
As molding material 21 is injected from injection port 22, the molding material forcefully impinges upon the end surfaces of the die pad 14 and semiconductor die 19, whereby they become twisted due to torsional force applied by the flowing molding material along the longitudinal axis of the tie bars 13. Such torsional force is proportional to the die pad area as well as the pressure applied to the injected molding material. The result is schematically shown in FIG. 6, which corresponds to a view taken along line 5--5 of FIG. 5. As seen, semiconductor die 19 and associated die pad 14 are twisted by an angle .theta. about the tie bars 13. If the stress distribution within the cured encapsulant 21 is uneven, a crack, such as indicated at 24, may be formed. Reference numeral 25 represents the outer leads.
Package cracking can result in lead wire rupture and/or moisture penetration. Once moisture accumulates in the package, rapid temperature increases (such as are experienced during solder reflow operations) will cause the moisture to vaporize and expand, thereby creating an internal pressure pocket, leading to delamination or further package cracking.
There is a need for semiconductor technology which avoids the above-mentioned problems and drawbacks attendant upon conventional packaging configurations, such as delamination. There also exists a need to simplify semiconductor packaging technology enabling to avoid the need for a different or customized lead frame design for each product, thereby reducing manufacturing costs.